1. Field of the Invention
The present invention relates to a wire-cut electric discharge machine having a function for automatic switching between a fixed temperature control mode and a differential temperature control mode.
2. Description of the Related Art
FIG. 5 is a schematic perspective view of a wire-cut electric discharge machine 30.
A predetermined low torque in the instructed direction reverse to the direction in which a wire electrode 20 is drawn out is applied by a torque motor 10 in a feeding section to a wire bobbin 11 having the wire electrode 20 wound therearound. While the wire electrode 20 unreeled from the wire bobbin 11 passes a brake shoe 13, a plurality of guide rollers, and a feed roller 19 driven by a wire electrode feeding motor (not shown), the tension of the wire electrode 20 between the brake shoe 13 and the feed roller 19 is regulated as the brake shoe 13 is driven by a brake motor 12.
After passing the brake shoe 13, the wire electrode 20 passes an upper wire guide 14, lower wire guide 15, and lower guide roller 16, then passes between a pinch roller 18 and the feed roller 19, and is collected into a wire electrode collection box 17. The workpiece to be machined (not shown) is disposed in a machining vessel 2.
In the wire-cut electric discharge machine 30, a voltage is applied across a gap between the wire electrode 20 and the workpiece to generate discharges for machining the workpiece. A machining fluid is provided between the wire electrode 20 and the workpiece for the purposes of insulation between the wire electrode 20 and the workpiece, cooling, and the removal of machining chips produced by the discharges. The temperature of the machining fluid is increased as the machining fluid is heated by the discharges between the wire electrode 20 and the workpiece. The temperature of the machining fluid is also increased by the heat loss from a pump supplying the machining fluid to the machining vessel 2 in which the workpiece is placed.
The increased temperature of the machining fluid thermally expands and deforms the workpiece and the table on which the workpiece is placed, causing the degradation of machining accuracy and the breakage of the wire electrode 20. The wire-cut electric discharge machine 30 is therefore provided with a cooling device for cooling the machining fluid.
The machining fluid in the wire-cut electric discharge machine 30 is cooled by the cooling device in the following two modes:
(1) Differential temperature control mode, in which a difference between the measured ambient temperature of the machine (wire-cut electric discharge machine) and the measured machining fluid temperature is detected and the machining fluid temperature is caused to follow the ambient temperature to limit the temperature difference to a preset range even if the ambient temperature changes, and
(2) Fixed temperature control mode, in which the machining fluid temperature is maintained at a preset temperature by the cooling device.
Japanese Patent Application Laid-Open No. 2007-203408 discloses a technique for controlling the temperature of a working fluid treating apparatus depending on the temperature of the machining fluid in the machining vessel in a wire-cut electric discharge machine. Japanese Patent Application Laid-Open No. 2007-319943 discloses a technique for controllably cooling the machining fluid on the basis of the temperature detected by a temperature detecting means that is selected from a plurality of temperature detecting means according to the machining conditions (whether the amount of heat generated by machining is large or small) of the wire-cut electric discharge machine and the target temperature.
With the method (1) above, a problem would arise when the ambient temperature of the machine widely varies, because the machining fluid temperature follows the ambient temperature and may readily cause thermal deformation of the machine due to the variation of ambient temperatures and the variation of machining fluid temperatures and degrade the machining accuracy. With the method (2) above, a problem would arise when the ambient temperature of the machine is substantially higher than the machining fluid temperature and the difference between the ambient temperature and the machining fluid temperature is large, because dew condensation may be formed on outer walls of the machining vessel in the machine, i.e., the wire-cut electric discharge machine.
The technique disclosed by the aforementioned Japanese Patent Application Laid-Open No. 2007-203408 takes into consideration in controlling the machining fluid temperature in the machining vessel neither the relationship (temperature difference) between the machining fluid temperature and the ambient temperature of the machine (wire electric discharge machine), nor prevention against the dew condensation on the machine in controlling the machining fluid temperature.
With the technique disclosed by Japanese Patent Application Laid-Open No. 2007-319943, a problem would arise when the temperatures widely vary between different temperature detection sites in the wire electric discharge machine, because a large temperature difference and thus dew condensation may readily arise when a different temperature detecting means is selected.